This disclosure relates generally to downhole tools and more specifically to tools for formation stress testing.
Downhole acquisition tools are used to extract quantitative information of formation rock stresses during certain drilling operations. The formation rock stress information may facilitate predicting geo-mechanical problems that may be associated with a wellbore of interest. For example, vertical stress, minimum horizontal stress, maximum horizontal stress, and azimuth of minimum horizontal stress are geo-stresses that may be used to characterize formation rock stress. These stress parameters may be estimated using various techniques. For example, vertical stress may be estimated from an integral of a density log obtained using the downhole acquisition tool. Minimum horizontal stress may be estimated using fracturing techniques and/or leak-off data, and its direction from borehole caliper or image analysis. During formation testing to estimate the minimum horizontal stress of the formation rock, the downhole acquisition tool injects a fluid into the formation to create a fracture. In particular, the downhole acquisition tool pumps a fluid into the formation, thereby causing a local increase in pressure at the injection site. The pressure continues to buildup until the formation rock mechanically fails and fractures. In certain instances, existing formation rock fractures may be reopened by injecting the fluid into the existing fractures. Following fracture, the injected fluid exits the fracture and a closure event (e.g., closing of the fracture) occurs. The minimum horizontal stress of the formation rock may be determined based on fracture closure pressure (e.g., the amount of pressure observed when the fracture closed).